Electro-migration is caused if current densities in an electrical wire become great enough to cause the atoms/ions that form the wire to migrate along the wire, i.e., high current density. Specifically, electrons or holes that conduct the current through the wire impart sufficient momentum to atoms/ions in the wire to cause migration of the atoms/ions. With the passage of enough current, the migration of atoms/ions along the wire causes the creation of voids in the wire and accumulation of excess material. The excess material is deposited elsewhere, often at corners in the wiring. The voids cause the current density around each void to increase, thus increasing the rate of electro-migration. The rate of electro-migration is also dependent upon the temperature of the wire. Voids that cause local increases in current density also increase the temperature of the wire due to ohmic heating, thus further increasing the rate of electro-migration. To reduce the effects of electro-migration, wiring of sufficient cross-sectional area is used to prevent an initiation of the electro-migration process. Because of the need to provide sufficient cross-sectional area, wiring on semiconductor chips often consumes large areas of the chip.
As the dimensions of silicon chips have become smaller, currents flowing along the wires have in general also decreased. Further, wiring materials on silicon chips have been modified over the years to accommodate high current densities without causing electro-migration. First, alloys of aluminum were used and more recently the wiring material has been changed to copper. Despite the above modifications, electro-migration is still an issue in high current density carrying parts of a chip. These high current density carrying parts include drivers for driving bond pads connected to wires outside of the silicon chip, power supply wiring on the silicon chip distributing power to the various devices forming the silicon chip and the clock circuit drivers to wires that distribute the clock signal across the silicon chip. In each of these places, high current densities occur and, at least in parts of the driver circuits, the current flow is in one direction. Current flow in one direction is particularly likely to cause electro-migration because the atoms/ions in the wire are constantly pushed in one direction. In wiring that has bidirectional current flow, for example, the clock wiring described above, the atoms/ions in the wire are pushed first one way then back again. Thus, although the atoms/ions move back and forth along the wire, in general, the wire degrades less quickly.